Storm Forecast
Valid: Thu 02 Jun 2011 06:00 to Fri 03 Jun 2011 06:00 UTC
Issued: Wed 01 Jun 2011 13:33
Forecaster: GATZEN

A level 1 was issued for north-western Russia mainly for large hail and severe wind gusts.

A level 1 was issued for western Belarus, eastern Poland, Slovakia, Hungary, south-western Ukraine and surroundings mainly for large hail and severe wind gusts.

SYNOPSIS

An intense frontal zone expands from central Scandinavia into western Russia. To the south, a high pressure area extends from the British Isles to south-western Russia. Southern Europe is affected by relatively low geopotential. An embedded cut-off low will slowly move eastward over the west Mediterranean.

DISCUSSION

North-western Russia

A strong south-westerly flow is forecast at the eastern flank of low geopotential extending from Iceland to Scandinavia. An embedded short-wave trough and associated jet streaks will move across the White Sea during the afternoon and evening. Along the frontal boundary that separates warm and rather dry air masses from cool maritime air, a frontal wave is expected to develop in response to the quasi-geostrophic forcing. Near the frontal boundary, rather rich low-level moisture will develop on Thursday, especially at the cold side where the boundary layer will be capped. Additionally, steep lapse rates are in place to the east of the frontal boundary.

Current thinking is that the frontal wave may lead to some warm air advection along the frontal boundary, especially to the east of the White Sea. The warm air and associated steep lapse rates may overlap with the rich low-level moisture just west of the surface cold front and is forecast to create CAPE in response to diurnal heating. As a result a favourable set-up for severe storms just west of the frontal boundary could evolve where the vertical wind shear will increase (15 m/s 3 km bulk shear).

Due to rather poor hodographs, multicells seem to be the main convective mode. Large hail and severe wind gusts as well as excessive rain are forecast with the stronger storms. Supercells are not ruled out with a threat of large hail. In the evening hours, increasing low-level vertical wind shear may also favour tornadoes, although the threat is expected to be low.

Farther east and south, decreasing vertical wind shear will limit the development of intense storms. However, storms that form may be capable of producing dry microbursts given the dry and well-mixed boundary layer.

East Europe

Steep lapse rates are present from western Russia to the Balkan states. CAPE is expected to build especially near the frontal boundary that stretches from western Russia across Belarus into central Poland. Rather rich low-level moisture is also expected over the northern Balkans. Diurnal heating and low-level convergence near the frontal boundary will likely assist for initiation during the day. The weak vertical wind shear will limit the severe potential, but a few stronger pulse storms may produce isolated large hail and excessive rain. Farther east, the threat of severe wind gusts will likely be more dominant given the well-mixed boundary-layer. Over the Balkans, rather strong easterly winds that will develop in the warm air advection regime in the evening hours may assist for some organized convection. So the threat of large hail and severe winds may increase over this area in the evening hours.

North-west Mediterranean region

Near the cut-off low, models expect a cool and dry boundary-layer that will likely limit the development of intense convection. The best potential of some stronger storms is expected over north-eastern Spain due to a low-level moist flow as well as near the Alps where some warm air advection will evolve in the afternoon hours.